Subsea wells offshore are typically developed using floating vessels to accommodate equipment, personnel, and operations necessary to drill and complete a well in order to initiate production of hydrocarbons from a given reservoir forming the target for the well. Additionally, testing and intervention work is typically executed through the use of such floating vessels. It is to be understood, however, that such a floating vessel also could be used in context of other types of subsea wells, for example water or gas injection wells.
It is understood that a floating vessel will be subjected to vertical movement due to the action of the waves of the sea (or a lake), which in turn introduces a challenge with respect to equipment utilized during operations carried out on the floating vessel. Such operations may include, but are not limited to, operations of drilling, completion, well testing, and well intervention. During operation at sea, said equipment will be subjected to vertical movement unless compensated for such movement.
As a floating vessel moves up and down in response to the waves, e.g. a drill string and a drill bit extending down below the vessel from a load-bearing structure, such as a top drive located within a drilling rig, will also move up and down. As it is essential that the weight on the drill bit, i.e. the downward force applied to the bit, is kept as constant as possible, such up and down movements of the drill bit are undesirable and provide for inefficient drilling progress, hence is counterproductive. Heave will remove weight from the drill bit as the rig moves up in conjunction with the high crest of a wave, while weight will be added to the drill bit as the rig moves down into the low point between two waves. Should hydrocarbons start to flow from a reservoir and into a wellbore being drilled, a valve arrangement is utilized to prevent such hydrocarbons from discharging into the natural environment and onto the floating drilling vessel. Such a valve arrangement is commonly referred to as a Blow Out Preventer (BOP), which is capable of sealing around, or cutting and sealing above, a drill pipe cut by shear rams in the BOP.
In other operations, which may include well testing and well intervention, e.g. wireline operations and coiled tubing operations, several sections of a high-pressure riser, commonly referred to as workover riser, are connected between equipment located at the seafloor, such as a subsea wellhead or a subsea Christmas tree, and the floating drilling vessel. The workover riser provides a barrier element for allowing control of pressurized hydrocarbon fluids present in the reservoir, and hence in the wellbore. A subsea valve arrangement, such as a subsea BOP, is also utilized in such operations to provide a system capable of sealing the well in case of an uncontrolled discharge of hydrocarbons from the reservoir. During such operations, hydrocarbon fluids may be present throughout the wellbore and the workover riser, and discharge at surface rig level is typically prevented by means of a valve arrangement located at surface, commonly referred to as a surface flow tree. A surface flow tree, or similar equipment attached to a workover riser, extending upwards from equipment located on the seafloor to the rig, is usually supported by, and kept in tension by, the top drive and drawworks forming part of the drilling rig on a floating drilling vessel. Various types of lifting equipment is utilized to connect the surface flow tree to the top drive, but also to hold the workover riser in tension as required to prevent high loads from acting on the equipment on the seafloor. Such lifting equipment may include, but Is not limited to, rigid bails, tension frames, and soft slings.
Well completion involves the use of production tubulars, which typically extend downwards from the wellhead and the Christmas tree to the producing zones bound by the reservoir(s) targeted by the well(s). Some parts of a completion operation will require equipment to be in tension in a manner similar to that described above. This may comprise setting the upper lock and seal mechanism of the production tubular, commonly referred to as a tubing hanger, inside the wellhead. At this point, a landing string, which is typically made up of several sections of drill pipe, will be connected to said tubing hanger at the wellhead, and also to the top drive at the floating drilling vessel via said lifting equipment. Similar to the description above, the weight of the system is controlled by holding said landing string in tension, thereby maintaining a known force at the level of said tubing hanger.
A vertical movement of a rig, as inflicted by waves of the sea, will impose tensional and compressive forces to said workover riser or landing string and accompanying equipment. These forces may be of a magnitude capable of fracturing or breaking such tubulars or equipment due to stress resulting from these forces. Such failure may, in turn, carry severe consequences, for example personnel injury and death, due to uncontrolled movement of equipment, or due to discharge of hydrocarbons to the surrounding environment, commonly referred to as a “blowout”, which may also result in permanent pollution to the natural environment.
In order to avoid such potential severe consequences, it is therefore critical to maintain a stationary position of the equipment and tubular strings discussed above with respect to a geodetic point, such as the seafloor. Hence, it is essential that the vertical movement of the rig is compensated for with respect to this stationary equipment when used for various well operations, for example drilling, completion, well testing, and well intervention. Based on this, all floating drilling vessels are equipped with a heave compensation system for ensuring that a load-bearing unit, such as a top drive, is heave-compensated. This implies that all equipment connected to the top drive, such as equipment located on the seafloor, is not unduly subjected to heave-related forces acting on the floating vessel. A functional heave compensation system is therefore critical to protect such equipment from the effects of heave-related, vertical movement of the floating vessel. Contrary, however, an inoperative and/or malfunctioning heave compensation system may allow for transmission of tensional and compressive forces to said equipment during various well operations, which in turn may result in severe consequences, for example failed equipment, personnel injury and death, and/or discharge of hydrocarbons to the environment (i.e. a “blowout”).
It would therefore be advantageous, or even critical in a harsh environment, to provide such a floating vessel with a backup heave compensation system capable of temporarily replacing the main heave compensation system should the main system become Inoperative and/or malfunction.